An attempt at a grand unified theory of filotimo.
Eleftherios V. Tserkezis touches on all the key aspects.
It is a Greek’s sense of honour, to use the old fashioned wording; of being respected in society, of social capital. It is what one can take pride in as an engaged member of society.
But this particular sense of honour is not tied up with chastity (as timi is): it correlates with discharging one’s social obligations; one gains honour by doing one’s duty. By meeting the social contract.
And one’s duty consists, to a large extent, of positive politeness strategies: of being considerate (hence the touchiness) and (more importantly) being generous.
There isn’t a simple translation, but I guess “being righteous” in some variants of English comes close.
An a-filotimos person, someone without filotimo, is ungrateful and/or ungenerous. There is a clear sense that, by doing so, this person violates the social contract.
EDIT: and filotimieme, to filotimo oneself, is to volunteer something as expected by the social contract. “He didn’t filotimo himself to offer any help” = “He didn’t bother offering any help, as the social contract demands”. This encompasses financial contracts too: an employee who isn’t doing their job conscientiously is also not doing filotimo.