Yours, Etc.: Origins and Uses of 8 Sign-Offs
a love note or letter, esp. sent by a secret admirer
The song serves as a "mash note" to his former girlfriend.
1880; fr. mash 'infatuation'
A very flattering letter, esp one proposing or offering sex: He gets mash notes by the ton/ showered him with embroidered pillows, mash notes, and cigars (1880s+)