I was wondering if there's an idiom or expression for a person who knows everything there is to know about their field of interest or expertise, practically no detail is unknown to them.

I thought of the expression: knowing something like the back of hand, but I guess that has a geographical sense to it and is used for cities, streets, or neighbourhoods. (Please correct me if I'm mistaken.)

  • Certainly "nerd" is sometimes used in this sense.
    – Hot Licks
    Commented Apr 10, 2021 at 0:21
  • In IT, and I suspect other areas as well, such people are sometimes known as "gurus". Of course the assumption that it is possible to know absolutely everything about a subject is rather flawed. The greatest experts in any given field all say that they are still finding things to learn. If nothing else they do research.
    – BoldBen
    Commented Apr 10, 2021 at 3:16
  • Are you looking for a word for the person (as it is boldfaced in your question), or something akin to "knowing like the back of someone's hand"? The tags suggest the latter, but your title the first.
    – Joachim
    Commented Apr 19, 2023 at 10:33

7 Answers 7


In this situation, I would say he "knows it backwards". The implication is that one knows something so thoroughly that they could do it in reverse.

Also consider

"knows it backwards and forwards"

{Cambridge Dictionary of American Idioms}

and "knows it inside out"

{McGraw-Hill Dictionary of American Idioms and Phrasal Verbs}.

"knows it like the back of his hand" does often, but not always, have a geographical connotation.

  • This answers OP's next question, not this one. Commented Oct 3, 2015 at 13:46
  • 1
    It answers the question as it was originally written (at the time that I wrote this answer). This question has been edited since then. Did I deserve a downvote for the question being edited after I answered it?
    – Holly
    Commented Oct 4, 2015 at 2:12
  • ' ... an idiom for someone who knows' (the original) equally demands a noun or equivalent. Commented Oct 4, 2015 at 14:09
  • @EdwinAshworth The original question also said "My own guess would be something like this: Since he developed this software framework himself, he knows all the words and words of it." Not sure how that would be interpreted as a request for a noun specifically.
    – Holly
    Commented Oct 4, 2015 at 23:12
  • 1
    You're right; OP invites related forms as well as noun groups. The repeat question is thus a duplicate. // I've edited to add attributions; this gives me the chance to retract the downvote. Commented Oct 5, 2015 at 18:54

That sounds like a "subject-matter expert (SME)."

A subject-matter expert (SME) or domain expert is a person who is an authority in a particular area or topic.

  • 1
    This answers OP's request inasmuch as it is a term for the person involved. But is it an idiom? Commented Oct 4, 2015 at 14:11
  • @EdwinAshworth: You're exactly right. I totally overlooked that an idiom was requested. I'm not sure if I should delete my answer or not. Do you think that is the proper protocol?
    – James
    Commented Oct 5, 2015 at 14:48
  • It's not an idiom, being totally transparent. But OP broadens with 'idiom or [other] expression' in the body of his post. Your answer looks OK to me; I'm really just asking about how common the expression is. Checking on the internet shows that it is far more common than I suspected. Don't delete; have an upvote instead. Commented Oct 5, 2015 at 19:10
  • I've since adjusted the title to correspond with OP's body question. Commented Mar 30, 2018 at 15:43

One idiom that people in the United States sometimes use to describe a person who possesses extensive, detailed knowledge of a particular subject is walking encyclopedia. Here is the entry for that term in Christine Amer, The American Heritage Dictionary of Idioms (1997):

walking encyclopedia A very knowledgeable person, as in Ask Ron—he's a walking encyclopedia of military history. A similar expression, a walking dictionary, was used by George Chapman in his poem "Tears of Peace" (c. 1600)


To know something like the back of one's hand.

To be intimately knowledgeable about something

'This professor is extremely knowledgeable, he knew Physics like the back of his hand.'


Given your example sentence, nuts and bolts is an idiom you could use.

Nuts and bolts: The basic practical details

Here are some examples of usage from Oxford Dictionaries Online:

The nuts and bolts of making a movie...

He tends to hire ambitious people who have their eye on a startup and a knack for the nuts and bolts of practice.

  • As you mentioned in your post, nuts and bolts refers to the basic details of something. I was looking for an idiom describing every possible detail. However, if you still think your answer is valid, please post it here.
    – Behdad
    Commented Oct 3, 2015 at 9:27
  • @Behdad: In that case, I think I'll let it slide. Thanks for the comment.
    – Tragicomic
    Commented Oct 3, 2015 at 11:43

Personally I think that has an intimate (or encyclopædic) knowledge of [field] is idiomatic per se.


Consider every jot and tittle

Every minor detail. A jot is the horizontal mark on the letter "t", and a tittle is the dot above an "i" or a "j".

Urban Dictionary

Also from soup to nuts

Also, from A to Z or start to finish or stem to stern . From beginning to end, throughout, as in We went through the whole agenda, from soup to nuts, or She had to learn a whole new system from A to Z, or It rained from start to finish, or We did over the whole house from stem to stern. The first expression, with its analogy to the first and last courses of a meal, appeared in slightly different forms (such as from potage to cheese) from the 1500s on; the precise wording here dates only from the mid-1900s. The second expression alludes to the first and last letters of the Roman alphabet; see also alpha and omega. The third comes from racing and alludes to the entire course of the race; it dates from the mid-1800s. The last variant is nautical, alluding to the front or stem, and rear or stern, of a vessel.

As noted, also from A to Z, from start to finish and from stem to stern.

American Hertiage Dictionary of Idioms at thefreedictionary.com

  • I like how jot and tittle sounds, but Google results are mostly about Bible. Is it a commonly used idiom? Considering the fact that no one else suggested this; is it an idiom the average English reader would understand?
    – Behdad
    Commented Oct 2, 2015 at 5:42
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    It is probably a bit archaic. I think most AmE speakers over 50 would know it, but younger, not so much. But that is just an impression.
    – bib
    Commented Oct 2, 2015 at 12:09
  • 1
    I know I messed up a little, but I think you should post your answer here.
    – Behdad
    Commented Oct 3, 2015 at 9:23

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