Origin: 1350–1400; ME (< AF) < L
aptus fastened, fitted, fitting, appropriate, equiv. to
ap- fasten, attach +
-tus ptp. suffix
Synonyms:1. liable.
2. See likely. 3. clever, bright; adaptable; handy, adroit, dexterous, skillful.
4. fitting, meet, germane, felicitous.
Apt, pertinent, relevant all refer to something suitable or fitting.
Apt means to the point and particularly appropriate:
an apt comment. Pertinent means pertaining to the matter in hand:
a pertinent remark. Relevant means directly related to and important to the subject:
a relevant opinion. Usage note:Some usage guides insist that
apt followed by an infinitive can or should be used to mean only “inclined, disposed”:
He is apt to ignore matters he regards as unimportant. In fact,
apt is standard in all varieties of speech and writing as a synonym for
likely in contexts that suggest probability without any implication of a natural disposition toward:
Hostilities are apt to break out if the confrontation is not soon resolved. She is apt to arrive almost any time now. See also liable. 
[apt] 