fibrillate v.

1680s, Englishing of Modern Latin fibrilla "a little fiber, a filament," especially in botany, diminutive of Latin fibra "a fiber, filament" (see fiber). Latin fibra and fibrilla were used in 17c. physiology in English alongside nativized fibre and fibril. From 1931 as "thread-like molecular formation."

verbal suffix for Latin verbs in -are, identical with -ate (1). Old English commonly made verbs from adjectives by adding a verbal ending to the word (such as gnornian "be sad, mourn," gnorn "sad, depressed"), but as the inflections wore off English words in late Old and early Middle English, there came to be no difference between the adjective and the verb in dry, empty, warm, etc. Thus accustomed to the identity of adjectival and verbal forms of a word, the English, when they began to expand their Latin-based vocabulary after c. 1500, simply made verbs from Latin past-participial adjectives without changing their form (such as aggravate, substantiate) and it became the custom that Latin verbs were Englished from their past participle stems.

1842, "state of being fibrillar" (that is, "arranged in fibrils"), noun of action from fibrillate (v.). Especially "a quavering in the fibrils of the muscles of the heart" causing irregular beating (1882).