Terminology
The term biceps brachii is a Latin phrase meaning "two-headed of the arm", in reference to the fact that the muscle consists of two bundles of muscle, each with its own origin, sharing a common insertion point near the elbow joint. The proper plural form of the Latin adjective biceps is bicipites, a form not in general English use. Instead, biceps is used in both singular and plural (i.e., when referring to both arms).
The English form bicep , attested from 1939, is a back formation derived from interpreting the s of biceps as the English plural marker -s. While common even in professional contexts, it is often considered incorrect.
The biceps brachii muscle is the one that gave all muscles their name: it comes from the Latin musculus, "little mouse", because the appearance of the flexed biceps resembles the back of a mouse. The same phenomenon occurred in Greek, in which μῦς, mȳs, means both "mouse" and "muscle".
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