means Covered in or shaped like sharp points, or by extension having a prickly, jagged, or hostile quality.

from From "spike," a sharp pointed object, plus the adjective ending "-y" that turns nouns into descriptions. "Spike" itself goes back to Middle English, likely borrowed from a Scandinavian source — Old Norse had words like "spik" and "spikr" for a nail or splinter — and it has cousins scattered across the Germanic family (think Dutch "spijker," a nail). So "spiky" is, quite literally, the quality of bristling with little nails.