word-forming element meaning "together, with," sometimes merely intensive; it is the form of com- used in Latin before consonants except -b-, -p-, -l-, -m-, or -r-. In native English formations (such as costar), co- tends to be used where Latin would use con-.

Proto-Indo-European root meaning "to hold firmly, support." 

It forms all or part of: affirm; confirm; Darius; dharma; farm; fermata; firm (adj.); firm (n.); firmament; furl; infirm; infirmary; terra firma; throne.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dharmah "custom, statute, law," dharayati "holds;" Prakrit dharaṇa "a holding firm;" Iranian dāra‑ "holding;" Greek thronos "seat;" Latin firmus "strong, steadfast, enduring, stable;" Lithuanian diržnas "strong;" Welsh dir "hard," Breton dir "steel."

c. 1300, confyrmacyoun, the rite whereby baptized persons are admitted to full communion with the Church, from Old French confirmacion (13c.) "strengthening, confirmation; proof; ratification," and directly from Latin confirmationem (nominative confirmatio) "a securing, establishing; an assurance, encouragement," noun of action from past-participle stem of confirmare (see confirm).

Meaning "verification, proof, supporting evidence" is from late 14c. Meaning "act of rendering valid by formal assent of authority" is from c. 1400; sense of "action of making sure, a rendering certain or proving to be true" from early 15c.

late 14c., of diseases, "firmly established," past-participle adjective from confirm. From mid-15c. as "supported by authority or proof." Of persons, "established in the habit, inveterate," from 1826.