coagulum.

Equivalent to cōgō (“gather together”) + -ulum (instrument noun suffix); the verb base in turn is equivalent to con- (“together”) + agō (“push, drive”), with the latter from the root *h₂eǵ-. The etymological origin of the suffix and of the lengthened vowel -ā- in the base is disputed; see -ulum and -ula. *One hypothesis derives the suffix from *-dʰlom and assumes *-ǵ-dʰlo- underwent regular cluster simplification with compensatory lengthening of the preceding vowel; thus, Sihler 1995 reconstructs Proto-Indo-European *h₂eǵ-dʰlo-. *De Vaan 2008 suggests alternatively that the long ā here and in ambāgēs could be analogical in origin, ultimately influenced by formations such as prōpāgēs and contāgēs (which may regularly continue the full-grade of *peh₂ǵ- and *teh₂g-, respectively). * Sen 2015 argues that the instrumental ending -ulum is more likely from the neuter of the suffix -ulus (found in some agent nouns, such as figulus (“potter”)) from Proto-Italic *-elos. *Nielsen 2004 considers it unlikely that instrument -ulum/-ula goes back to either *-dʰlom or agentive *-(e)lo-, and proposes it developed from *-tlom (> Proto-Italic *-klom), which caused the vowel of the preceding syllable to be lengthened by Lachmann's Law. Afterwards, voiced /ɡ/ was reintroduced either by analogy to the base verbs, or by a conditioned sound change that turned /k.l/ across morpheme boundaries to /ɡl/ after a long vowel. Accordingly, Nielsen reconstructs pre-Latin *kom-ag-klo-.