Feeling buried or swamped by something.
Feeling buried or swamped by something — emotion, demands, or sensation — that exceeds one's capacity to cope.
The most quantitative of this cluster: the felt sense is of capacity exceeded, of being submerged by sheer volume — too much to do, too much feeling, too much at once. It runs deeper and heavier than mere stress, and unlike flustered or rattled it need not involve a loss of poise; one can be quietly, privately overwhelmed. Note its genuine two-sidedness: while the default modern sense is the crushing, stressful one (overwhelmed by work, by grief), it is just as idiomatic in a positive flood — overwhelmed with gratitude, joy, or love — where the same submerging force is welcome rather than punishing. It is heavier and more enveloping than apprehension or agitation, and broader than panic, which is sharper and more acute.
From the verb overwhelm, mid-14c. overwhelmen "to turn upside down, overthrow, knock over," from over- + Middle English whelmen "to turn upside down" (the root "whelm," early 14c., probably from or altered by Old English helmian "to cover"). The adjective "overwhelmed" is recorded from mid-15c., originally "completely submerged or swamped." (Merriam-Webster dates the adjective to the 15th century.)
The word's root sense is starkly physical — to "turn upside down" or "overset," the connecting image (per Etymonline) being a boat washed over and capsized by a big wave. By early 15c. it had broadened to "submerge completely," and the figurative "bring to ruin" sense is attested from the 1520s. From this literal overturning/submerging the modern emotional meaning grew naturally: to be overwhelmed is still to be metaphorically flooded or sunk by force, numbers, thought, or feeling. The dual valence (crushing vs. joyful flooding) is well established in current usage.