Classical Armenian aorists are traditionally divided into strong (root) and weak, the latter characterized by a stem-final -cʻ-. The great majority of weak aorists to presents in -em and -im take the suffix -eacʻ ~ -ecʻ- (e.g. sirecʻi, 3sg. sireacʻ 'loved' to pres. sirem), but a small group are in -acʻ- (e.g. asacʻi 'said' to pres. asem).
Weak aorists to presents in -am always take the suffix -acʻ- (e.g. yusacʻay 'hoped', luacʻi 'washed' to pres. yusam, luanam), while a small set have a bare -cʻ- following the root (e.g. lcʻi 'filled', 3sg. elicʻ). The origin of these suffixes has been debated for over a century, but no satisfactory explanation of their shape and distribution has yet been proposed.
It is argued that the weak aorist suffix goes back to *-ā-, in origin a (post-)PIE optative in *-e-h2-, and is cognate with Balto-Slavic preterite *-ā- and Italic imperfect *-ā-. The productive denominative and factitive presents in PIE *-eye/o- thus formed an aorist in *-eyā-.
Contrary to received opinion, unlike vowels which came into contact following the loss of intervocalic *y did not contract, so the sequence *-eyā- yielded the diphthong *e‿a and spread to the aorists of almost all presents in -em, leaving only a small relic group in -acʻ-. The *-ch- of the weak aorist marker continues PIE *-sḱe/o- and was generalized from iterative-intensive imperfects to all imperfect-aorists, before the creation of the new imperfect in -i/y-.
These hypotheses are integrated into a unified model of the evolution of the Classical Armenian verb that also accounts for the relic type of berem 'carry', aor. beri and the distribution of strong and weak aorists to nasal presents, e.g. erduay 'feared' (pres. erdnum) vs. lcʻi 'filled' (pres. lnum).