IPA: /ˈmoʊtɪv.ləs.li/
KK: /ˈmoʊtɪvlesli/
In a way that lacks a reason or purpose.
He acted motivelessly, leaving everyone confused about his intentions.
Motivelessly → It is formed from "motive" (from Latin "motivus", meaning causing motion) and the suffix "-less" (meaning without) combined with the adverbial suffix "-ly". The word "motivelessly" means without any cause or reason for action.
Think of being 'without' ('-less') a 'cause for action' ('motive') — that's why motivelessly means acting without a reason.