expectancy n.

1550s, "wait, defer action," from Latin expectare/exspectare "await, look out for; desire, hope, long for, anticipate; look for with anticipation," from ex- "thoroughly" (see ex-) + spectare "to look," frequentative of specere "to look at" (from PIE root *spek- "to observe").

The figurative sense of "anticipate, look forward to" developed in Latin and is attested in English from c. 1600. Also from c. 1600 as "regard as about to happen." Meaning "count upon (to do something), trust or rely on" is from 1630s. Used since 1817 as a euphemism for "be pregnant." In the sense "suppose, reckon, suspect," it is attested from 1640s but was regarded as a New England provincialism. Related: Expected; expecting.

word-forming element denoting quality or state, from Latin -antia, forming abstract nouns on past-participle adjectives in -antem, appearing in English mostly in words borrowed directly from Latin (those passing through French usually have -ance or -ence; see -ance). But English also has kept many pairs of words in -ance and -ancy (radiance/radiancy, etc.). Though typically one of the two forms has been more common, both were kept "as metrically useful or rhetorically effective" [Fowler, 1926].