"of or belonging to an apex," 1827, from Latin apicem, from apex (see apex) + -al (1).
Entries linking to apical
apex n.
"the tip, point, or summit" of anything, c. 1600, from Latin apex "summit, peak, tip, top, extreme end;" which is plausibly related to apere "to fasten, fix," hence "the tip of anything" (one of the meanings of apex in Latin was "small rod at the top of the flamen's cap"), and thus ultimately from PIE *ap- (1) "to take, reach" (see apt). But if the original notion was "point," not "top," it might go another way. Proper plural is apices.
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).