Turns out that this word once had a significantly different pronunciation than it does today. An interesting case where a word with separate forms for verb and noun transformed into a single form for both. I believe I tracked down the “O.P.” referred to below, but I’m not certain. I think it is “O.P. now hist., old price(s), referring to demonstrations at Covent Garden Theatre, London, in 1809, against a proposed new tariff of prices.” And, as before, the asterisks stand-in for symbols Blogger will not reproduce.
[OE. æce is a primary deriv. of vb. ac-an to ACHE, in which, as in parallel forms, the c (k) was palatalized to ch (t*), while in the vb. it remained (k); cf. make, match; bake, batch; wake, watch; break, breach; speak, speech; stick, stitch. Occasional early instances of ake as n. are northern, in which dialect c (k) was not palatalized, cf. make = match, steik = stitch, kirk = church. In 7 the n. was still atche (**t*, **t*) pl. atch-es (**t**z, **t**z), but about 1700 it began to be confused with the vb. as (**k). The spelling of the latter has in turn been changed to ache, so that though both vb. and n. are now really ake, both are in current spelling written ache. See prec. The former pronunciation survives in the dialectal eddage = head-ache; cf. Smallage for Small Ache f. ACHE n.2 The ‘O.P.’ rioters, ignorant of the Shaksperian distinction of ake and ache, ridiculed the stage pronunciation of the n. by giving it to the vb. in ‘John Kemble’s head aitches’.]
A pain; in later usage, a continuous or abiding pain, in contrast to a sudden or sharp one. Used of both physical and mental sensations.