crazy adj.

late 14c., crasen, craisen "to shatter, crush, break to pieces," probably a Germanic word and perhaps ultimately from a Scandinavian source (such as Old Norse *krasa"shatter"), but it seems to have entered English via Old French crasir (compare Modern French écraser). Original sense preserved in crazy quilt (1886) pattern and in reference to cracking in pottery glazing (1815).

Mental sense of "derange the intellect of, make insane" (late 15c.)  perhaps comes via the transferred sense of "be diseased or deformed" (mid-15c.), or it might be an image of cracked or broken things. The intransitive sense of "become insane" is by 1818.  Related: Crazed; crazing.

... there is little assurance in reconciled enemies: whose affections (for the most part) are like unto Glasse; which being once cracked, can neuer be made otherwise then crazed and vnsound. [John Hayward, "The Life and Raigne of King Henrie the IIII," 1599]

adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy). Adjectives such as hugy, vasty are artificial words that exist for the sake of poetical metrics.