Akon -- Sued in $39,000 Lawyer Battle

Akon's in the process of learning a very valuable lesson -- don't piss off your lawyers ... because they're experts in legal ass-beatings.

According to a new lawsuit, filed in Georgia, the "Smack That" singer hired a bunch of lawyers last year at a firm called Yoss -- then refused to pay them $39, 363.74 for legal services. Now, they've responded by doing what they do best ... suing his pants off for breach of contract.

It's not entirely clear what the legal work entailed -- but sources tell TMZ, it involved civil/business matters.

Akon filed his own response in the suit -- insisting their claims are BS ... and vowing to fight back.