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1550s, "wait, defer action," from Latin expectare/exspectare "await, look out for, desire, hope," from ex- "thoroughly" (see ex-) + spectare "to look," frequentative of specere "to look at" (see scope (n.1)).
Figurative sense of "anticipate, look forward to" developed in Latin, attested in English from c.1600. Used since 1817 as a euphemism for "be pregnant." Related: Expected; expecting.