"the penis," 1530s (mid-13c. as a surname), dialectal variant of Middle English pil-cok, pillicock "the penis" (see cock (n.3)). Meaning "stupid person" is attested by 1967.
pillar
pillbox
pillbug
piller
pillion
pillock
pillory
pillow
pillowy
Pilobolus
pilon
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