Sometimes, when engaged in conversation, English-speakers will say the first half of a well-known saying or idiom, trailing off at the end or punctuating it with a shrug. For instance, they might say "Birds of a feather..." or "When in Rome..." and leaving the "flock together" and "do as the Romans do" completely unsaid, when arguably that unspoken portion was the whole point of their statement in the first place.
Is there a name for such abbreviated usage?
The technical term, anapodoton is an appropriate, and perhaps more accurate alternative to ellipsis
An anapodoton (from the Greek anapodosis: "without a main clause") is a rhetorical device related to the anacoluthon. It is a figure of speech or discourse that is an incomplete sentence, consisting of a subject or complement without the requisite object. The stand-alone subordinate clause suggests or implies a subject (a main clause), but this is not actually provided.
As an intentional rhetorical device, it is generally used for set phrases, where the full form is understood, and would thus be tedious to spell out, as in “When in Rome [do as the Romans].” or “If the mountain won't come to Muhammad [Muhammed will go to the mountain].”
You might also call them clipped, or simply, shortened idioms. Here is a list of shortened idioms that I found scouring the ELU archives.
great minds
derived from Good wits doe jumpe, which transformed into
Great minds think alike
Pot, kettle, black
Derived from The pot calls the pan burnt-arse, which later became
The pot calling the kettle black
devil-may-care
Originally: The devil may care, but I do not
second nature
derived from an ancient proverb: Custom (or usage) is a second nature
speak of the devil
derived from Speak of the devil and he doth appear
the last/final straw
originally: The straw that broke the camel's back
the more the merrier
derived from: The more the merrier; the fewer, the better fare
when in Rome
Originally: When in Rome, do as the Romans do
every man for himself
Originally: Every man for himself, and the Devil take the hindmost
I found a noble page with many rhetorical devices described. Included is the paradoxical Inclusion by Omission:
There are two words that may apply.
aposiopesis
An unfinished thought or broken sentence. "If you don't eat up that spinach, I'll..."
ellipsis
Omission of one or more words. "If only pigs could fly!"
Please also see this Wikipedia page on Ellipsis (linguistic).
In linguistics, ellipsis (from the Greek: ἔλλειψις, élleipsis, "omission") or elliptical construction refers to the omission from a clause of one or more words that are nevertheless understood in the context of the remaining elements.
