These days, someone alleges something before presenting the evidence to prove it (or perhaps without evidence at all), but the word actually derives from the Middle English verb alleggen, meaning "to submit (something) in evidence or as justification." Alleggen, in turn, traces back to Anglo-French and probably ultimately to Latin allegare, meaning "to send as a representative" or "to offer as proof in support of a plea." Indeed, allege once referred to the actions of someone who came forward to testify in court; this sense isn't used anymore, but it led to the development of the current "assert without proof" sense.