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"glance" (especially through a small opening), mid-15c., perhaps alteration of Middle English piken (see peek (v.)). Peeping Tom "a curious prying fellow" [Grose] is from 1796; connection with Lady Godiva story dates only from 1837.
"make a short chirp," c.1400, probably altered from pipen (mid-13c.), ultimately imitative (cf. Latin pipare, French pepier, German piepen, Lithuanian pypti, Czech pipati, Greek pipos).
1520s, first in sense found in peep of day, from peep (v.1); meaning "a furtive glance" is first recorded 1730.
"short chirp," early 15c., from peep (v.2); meaning "slightest sound or utterance" (usually in a negative context) is attested from 1903. Meaning "young chicken" is from 1680s. The marshmallow peeps confection are said to date from 1950s.
positive end-expiratory pressure