means A short metal pin or bolt used to fasten two pieces of material together, hammered or pressed flat at the headless end so it can never simply slip back out.

from From Middle English via Old French 'rivet,' from the verb 'river' meaning to fix or clinch — likely related to driving something firmly into place. The figurative sense, to 'rivet' someone's attention, comes straight from the image: just as a clinched rivet cannot pull free, a riveted gaze is pinned and held fast.