"things contained, that which is contained" in something (the stomach, a document, etc.), early 15c., from Latin contentum (plural contenta), noun use of neuter past participle in the literal sense of continere "to hold together, enclose," from assimilated form of com "with, together" (see con-) + tenere "to hold" (from PIE root *ten- "to stretch"). Table of contents is from late 15c.
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-.