Why is the ninth month called September?
early 13c. from sleep (n.) + -y (2). Perhaps in Old English but not recorded. Old English had slæpor, slæpwerig in the sense "sleepy;" slæpnes "sleepiness." Cf. Old High German slafag. Of places, from 1851 (Irving's Sleepy Hollow is from 1820). Sleepy-head is from 1570s. Related: Sleepily; sleepiness.