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.

*mē-, Proto-Indo-European root meaning "to measure." Some words may belong instead to root *med- "to take appropriate measures."

It forms all or part of: amenorrhea; centimeter; commensurate; diameter; dimension; gematria; geometry; immense; isometric; meal (n.1) "food, time for eating;" measure; menarche; meniscus; menopause; menses; menstrual; menstruate; mensural; meter (n.1) "poetic measure;" meter (n.2) unit of length; meter (n.3) "device for measuring;" -meter; Metis; metric; metrical; metronome; -metry; Monday; month; moon; parameter; pentameter; perimeter; piecemeal; semester; symmetry; thermometer; trigonometry; trimester.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit mati "measures," matra "measure;" Avestan, Old Persian ma- "to measure;" Greek metron "measure," metra "lot, portion;" Latin metri "to measure."

"not of equal measure; not having a common measure," 1640s, from in- (1) "not, opposite of" + commensurate.

c. 1300, mēte, "having the right shape or size," from Old English gemæte, Anglian *gemete, "suitable, having the same dimensions," from Proto-Germanic *ga-mætijaz (source also of Old Norse mætr, Old High German gimagi, German gemäß "suitable"), from collective prefix *ga- + PIE root *med- "take appropriate measures." The formation is the same as that of commensurate. Meaning "proper, appropriate" is from early 14c.; that of "fit (to do something)" is from late 14c.

The mountain sheep are sweeter,

But the valley sheep are fatter;

We therefore deem'd it meeter

To carry off the latter.

[Thomas Love Peacock, from "The War-song of Dinas Vawr"]