Douglas Harper's Etymology Dictionary
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 (compare Gaulish beccus "beak").
pre. In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10−12. Symbol: p
Pico- (symbol p) is a unit prefix in the metric system denoting one trillionth, a factor of 10 .
Derived from the Spanish , meaning peak, beak, bit, this was one of the original 12 prefixes defined in 1960 when the International System of Units was established.