complain v.

word-forming element usually meaning "with, together," from Latin com, archaic form of classical Latin cum "together, together with, in combination," from PIE *kom- "beside, near, by, with" (compare Old English ge-, German ge-). The prefix in Latin sometimes was used as an intensive.

Before vowels and aspirates, it is reduced to co-; before -g-, it is assimilated to cog- or con-; before -l-, assimilated to col-; before -r-, assimilated to cor-; before -c-, -d-, -j-, -n-, -q-, -s-, -t-, and -v-, it is assimilated to con-, which was so frequent that it often was used as the normal form.

*plāk-, Proto-Indo-European root meaning "to strike."

It forms all or part of: apoplexy; cataplexy; complain; fling; paraplegia; plaint; plangent; plankton; planxty; plague; plectrum; quadriplegia.

It is the hypothetical source of/evidence for its existence is provided by: Greek plazein "to drive away," plēssein "to beat, strike;" Latin plangere "to strike, lament;" Old English flocan "to strike, beat;" Gothic flokan "to bewail;" German fluchen, Old Frisian floka "to curse."