c.1400, originally in reference to the presence of Christ in the Eucharist, from real (adj.) + -ly (2). Sense of "actually" is from early 15c. Purely emphatic use dates from c.1600; interrogative use (oh, really?) is first recorded 1815.
re-ally
v.
"to form an alliance again," c.1600, from re- + ally (v.). Related: Re-allied.