Firma versus Poderosa

In Chilango slang, firmar (feer-MAR, lit. to sign) or echar la firma (eh-CHAR lah FEER-mah, lit. to throw the signature) means to urinate. By splashing urine one leaves a distinctive mark, similar to, but more primal than, a John Hancock. If you are in Mexico City and you actually mean to sign, the appropriate term is echar la poderosa (poh-the-ROSS-ah, lit. to throw the mighty one). One's poderosa is one's signature.

Ejecutivo de cuenta: Sr. Menganito, nada más nos falta su poderosa, y el efectivo le será entregado en la caja 3.
Sr. Menganito: En cuanto termine de echar la firma, si me permites.

Account manager: Mr. Menganito, we only need your signature and you'll get the cash at teller number 3.
Mr. Menganito: As soon as I am done here, if you don't mind.