accipiter.

Uncertain. The term parallels other formations in other Indo-European languages, such as Proto-Slavic *àstrę̄bъ, Sanskrit आशुपत्वन् (āśupátvan), and Ancient Greek ὠκύπτερος (ōkúpteros, “swift-winged”). De Vaan argues that the first part of the compound, ac-, possibly derives from Proto-Indo-European *HéHḱus (“swift, quick”). Alternatively, the philologist Claire Le Feuvre argues that this component probably derives from Proto-Indo-European *h₂eḱ-, whence also acus. The second part of the compound, -piter, possibly derives from earlier *petro- or *petri-, both of which—according to De Vaan—may have meant "wing." This second component is most likely connected to Proto-Indo-European *peth₂- (“to spread out, fly”). De Vaan provides a Proto-Italic pre-form *aku-petris (“having swift/pointed wings”). Based on the comparison with Ancient Greek, Beekes suggests a possible Proto-Indo-European pre-form *HHḱu-petro- (“fast-flying”). According to Nussbaum, the connection with Ancient Greek πτερόν (pterón) requires an earlier Latin form *aci-pteri-, which would have evolved into accipiter via anaptyxis. Nussbaum argues that the reconstruction *-petris for the second component is preferable to ensure consistency with Sanskrit पत्र (pátra). Nevertheless, Nussbaum still considers the latter reconstruction problematic, arguing that Plautine accipetrina attests to a continuation of the original vocalism. Instead, Nussbaum opts for a reconstruction *aci-peter- based on a comparison with Hittite pát-tar. The geminate -cc- is perhaps influenced by accipiō (“take, seize”). Alternatively, it has been explained as a development from earlier *āci- via the littera-rule. Though, Nussbaum rejects the second possibility as it would be the only known example in which the littera-rule affected a vowel that was not long -ī- or -ū-.