Thread:User talk:CodeCat/West Germanic infinitive suffix/reply

The Middle Dutch etymology really says all that can be said as far as I can see. The infinitive suffix -en in German doesn't have a single definitive etymology. Originally, the infinitive suffix was just -ną in Proto-Germanic. Various vowels would appear before that suffix depending on the type of verb, giving -aną (strong verbs), -janą (class 1 weak verbs), -ōną (class 2 weak verbs), -āną (class 3 weak denominatives), -janą (class 3 weak statives), -naną (class 4 weak). Most of them were still distinct in OHG: -an, -(i)en, -on, -en, -non respectively. But because unstressed vowels became schwa in the transition to MHG, all of the suffixes fell together into a single common -en and were no longer distinguished. So the answer to "what is the origin of the German infinitive suffix -en" is "all of them!".