Meaning

noun an arrangement of two or more objects or persons one behind another

noun a bicycle with two sets of pedals and two seats

adverb one behind the other

originPIE word *téh₂m The noun is borrowed from Latin tandem (“of time: at last, at length, finally”), applied humorously in English to two horses harnessed “at length” (that is, in a single line) instead of side-by-side. Tandem is derived from tam (“so, to such an extent”) + -dem (demonstrative suffix). The adjective, adverb, and verb are derived from the noun.

Etymology from Wiktionary, CC BY-SA 3.0