Harry's blues site has the following definition of 'Signifying':

1 - a good-natured needling or teasing especially among urban blacks by means of indirect teasing with taunting words and clever, often preposterous "put-downs" (humiliating remarks); 2 - In signifying, speakers spontaneously compose rhythmic and rhyming phrases in improvised counterpoint to the signified phrases of other speakers. Within this word play structure, signifying is an indirect speech act form that allows the speaker to express bold ideas, opinions, beliefs, or feelings without repercussions as the stated convictions become diffused through the playful nature of the act. This improvisational verbal device arose as a component of the call and response form and became incorporated into blues lyrics.

'Shave 'em dry' seems to stem from an old bawdy song, primarily favoured by women blues singers and first recorded by Ma Rainey. The best-known version is by Lil Johnson wherein she 'signifies' by making slanderous accusations of a sexual nature. Lucille Bogan (recording as Bessie Jackson) had an extremely raunchy version with lyrics such as the following:

I got nipples on my titties big as the end of my thumb
I got somethin''tween my legs'll make a dead man come
Oooh daddy - baby, won't you shave 'em dry, oooh!
Won't you grind me baby, grind me till I cry

The full text of the Johnson and Bogan versions may be found in Paul Oliver's 'Screening the Blues'. Two versions by Bogan may be found on 'Raunchy Business: Hot Nuts & Lolly Pops' Sony Roots N'Blues Series Columbia CK 46783.

Oliver notes that the term has several layers of meaning in that, at one level it refers to mean and aggressive action but, as a sexual theme, it refers to intercourse without preliminary lovemaking. Big Bill Broonzy: 'Shave 'em dry is what you call makin' it with a woman; you ain't doin' nothin', just makin' it'. Because the implications of the term were of pubic contact, it could be applicable to either sex. [Info from Paul Oliver 'Screening the Blues'Da Capo Press p225]

Hope the above is of some use.

--Stewie.