combustion (n.)

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.

"small, live coal," Old English æmerge "ember," merged with or influenced by Old Norse eimyrja, both from Proto-Germanic *aim-uzjon- "ashes" (source also of Middle Low German emere, Old High German eimuria, German Ammern); a compound from *aima- "ashes" (from PIE root *ai- (2) "to burn;" see edifice) + *uzjo- "to burn" (from PIE root *heus- "to burn;" source also of Sanskrit osati "to burn, scorch," usna- "hot;" Greek euo "to singe;" Latin urere "to burn, singe;" Old Norse usli, Old English ysle "hot ashes," Old Norse ysja "fire"). The -b- is unetymological.