It's probably slang for "line of work: occupation":

https://www.merriam-webster.com/dictionary/lay

See 3b under its noun definition.

So: “Why, sir, in a manner of speaking: but some might say more on the porcupine-lay, the roving-line, if you understand me.”

Translates to: “Why, sir, in a manner of speaking: but some might say more on the concubine-occupation, the roving-line [of work], if you understand me.”

Then Jack, not wanting to embarrass Heaven for his malapropism, intentionaly reuses "porcupine" incorrectly.