The clue to 48-down was: "Muscle strengthened by curls, informally." The answer: BICEP. And the prescriptivist dog whistle in the clue is the word "informally."
As Miss Mossman explained years ago in Latin class, biceps is a singular, though it's long been used as the English plural too. (Fowler liked bicepses better than the Latin bicipites, but neither caught on.) So according to traditionalists, the word bicep shouldn't exist. Like pea and cherry and kudo, it was formed on the erroneous assumption that a final-s sound signaled a plural.
But the advance of singular bicep was so stealthy that few language mavens noticed it over the years. By the time Bryan Garner got into the usage trade, it was so well established that he simply accepted it. In the most recent (2009) edition of Garner's Modern American Usage, he says that "to refer to a person's right biceps ... seems pedantic." Despite etymology, "the standard terms are now bicep as the singular and biceps as the plural."
I'm not so sure singular biceps has been relegated to nonstandard (or even "pedantic") status, and neither is Google Books, despite that ominous post-2000 drop on the Ngram chart. But "right bicep" is obviously acceptable to many, even if some of us still think of it as "informal." Sorry, Miss Mossman!