Apt means exactly suitable, or likely to do something. An apt description, apt to forget, an apt pupil.
Of a word or comparison it means precisely fitting: an apt phrase, aptly named. Followed by to it means inclined or prone — apt to rain, apt to lose his temper — which is a slightly formal and rather old-fashioned use. An apt pupil is a quick one, which is the sense that survives in aptitude. Note aptitude, aptly; and that inapt and inept are different words with the same ancestor.
1adjectiveaptExactly suitable.
“The nickname "the Professor" was apt for such a clever, serious boy.”
synonyms: fitting, appropriate
2adjectiveinclinedLikely or inclined to do something.
“He's apt to lose his temper when he's tired.”
apt is a quality of fitness or tendency.
Latin aptus meant fitted or joined, from a root meaning to fasten. Adapt, aptitude and attitude all come from it, so an apt phrase is one that fits.
Latin
aptus — suitable, fitting
Old French
apte
English, today
apt
Built from
From Old French apte, from Latin aptus (“suitable, fitting”). Either from obsolete apio (“to fasten, to join, to fit”), akin to apisci (“to reach, attain”) (compare with Greek ἅπτειν (“to fasten”) and Sanskrit आप्त (“fit”), from आप् (“to reach, attain”)) or from h₂ep- (“to join, fit (in)”).
Etymology from Wiktionary, CC BY-SA