What is the meaning of -um-?

an agent trigger infix

When placed before the first vowel, forms intransitive, actor focus verbs

antonym of ag-: implies the action is of less duration or singular

implies the possibility of the action to take place

indicates residents or inhabitants of a place

alternative spelling of -om-

intransitive verb affix

inflectional affix marking irrealis mood with transitive verbs

active verb

suffix to be used only in specific cases where the meaning is also specified

actor trigger: to do something

no trigger: expresses natural phenomena

actor trigger: to become or get

actor trigger: (with root word reduplication) to do something occasionally, at random, a little, a bit, now and then or here and there

Source: wiktionary.org