We do have this idiom:
like water off a duck's back
You say that criticism is like water off a duck's back or water
off a duck's back to emphasize that it is not having any effect on
the person being criticized.
In Endangered Phrases: Intriguing Idioms Dangerously Close to Extinction, author Steven D. Price ...
As you say, if the cap/shoe fits, wear it does not mean the same thing as the negated statement if it's not your shirt, don't put it on. This particular translation appears to be a common problem among Hungarian - English free online dictionaries (EUdict, bab.la), which may be borrowing from similar sources of information like this 2006 Hungarian English ...
As Redd Foxx, the late comedic actor, was known to say, "Beauty is only skin deep, but ugly is to the bone."
Here are some words to describe a handsome or beautiful person who does not have the charisma or personality to match. (Not all the words convey the idea of beauty in combination with a lack of charisma):
These all carry the implication that the person is primarily important for their looks. It doesn't say that they explicitly lack personality or charisma, more that they're not relevant.
Collins dictionary :
Eye candy is used to refer to people or things that are attractive to look at but are not interesting in other ...
be just a pretty face.
To be physically attractive but lack any distinguishing achievements, intelligence, abilities, or other personal characteristics.
Tiffany might be popular because of her looks now, but once we're out
of college she's going to be just a pretty face.
(The Free Dictionary)
Actually, "Homeopathed" isn't that far off. In theory a "homeopathic" preparation is one where the original drug/herb/whatever is so diluted that there may not even be a single molecule of the stuff in the resulting preparation. The argument is that the water "remembers" the presence of the stuff, even after it's gone.
Something may be faded out, literally or figuratively.
fade out 1. PHRASAL VERB
When something fades out, it slowly becomes less noticeable or less
important until it disappears completely. [Collins CoBuild]
fade out ...
to diminish and go away altogether. [McGraw-Hill Dictionary of
American Idioms and Phrasal
You've so phrased your question that it conveys more than one interpretation. In Chemestry you might be looking for the word "solution" or "dissolved"?
solution a liquid mixture in which the minor component (the solute) is uniformly distributed within the major component (the solvent).
dissolve (with reference to a solid) become or cause to become ...
For both scenarios you describe, I would quote former US representative from Oklahoma J. C. Watts, to say it's a clear case of having no character.
Character is doing the right thing when nobody's looking. There are too many people who think that the only thing that's right is to get by, and the only thing that's wrong is to get caught. Watts
If you rephrase the question and ask "why do we behave better when we're being observed?" then the expression would be 'social pressure', 'peer pressure' or 'group pressure' depending on the situation.
Psychology Dictionary definition for Group pressure:
any direct or indirect social pressure that is exerted by a group on its individual members to ...
"Synergistic" comes to mind -
Working together; used especially of groups, as subsidiaries of a
corporation, cooperating for an enhanced effect.
The interaction of two or more agents or forces so that their combined effect is greater than the sum of their individual effects.
“Sticks in a bundle strengthen each other.” (”In unity is strength.”)
The Bundle of Sticks
A certain Father had a family of Sons, who were forever quarreling among themselves. No words he could say did the least good, so he cast about in his mind for some very striking example that should make them see that discord would lead them to ...
In psychology this is a special feature of social change called the Snowball effect, whereby the minority group influences or becomes the majority group. The minority group can start as an individual, before slowly growing, being heard and eventually being accepted into the “majority group”:
Social change occurs when a whole society adopts a new belief or ...
Union is strength comes to mind:
This proverb means that ‘sticking together is a source of strength’. If people join together, they are more powerful than if they work by themselves.
(The Free Dictionary)
We could say, in your example, "Hassim must have food on the brain!" This means that his current thoughts are of food and affecting his other actions, not necessarily that he has a one-track mind for food all the time. (One-track mind can be used with for, as I just did, but usually it is simply stated that someone "has a one-track mind" and the focus is ...
The word sucker seems to fit the bill. Chambers online provides the definition
someone who is gullible or who can be easily deceived or taken
The word doesn't mean that the person's character always fails them which is what is asked for in the question. But the trail of commentary clarifies that sucker covers at least most of what OP is ...