means Feeling buried under more demands, emotions, or work than you can manage at once.

from From Middle English 'whelmen,' to capsize or cover over — picture a boat 'whelmed' by a wave, turned upside down and swallowed. Add the intensifying 'over-' and you get the sense of being completely engulfed. The word started literally watery, describing things drowned or buried, before it sank into the realm of feelings, where most of us now meet it.