What does Boxing Day have to do with boxing?
word-forming element used in making names for very small unites of measure, 1915 (formally adopted c.1952 as a scientific prefix meaning "one trillionth"), from Spanish pico "a little over, a small balance," literally "sharp point, beak," of Celtic origin (cf. Gaulish beccus "beak").
One-trillionth (10-12): picogram.
Very small: picornavirus.