Bill Cowher -- I'm Grandpa Now ... to a Baby Laker

'Coach' and 'The Chin' weren't enough ... so now Bill Cowher can also go by 'Grandpa' -- his daughter Lindsay and her husband, Lakers forward Ryan Kelly, just had their first baby.

The baby ... Nile Jeremiah Kelly was born last night, and he's a big boy ... 8.8 lbs. and 22 in. long.

It's not known if Coach Cowher was actually at the hospital when Lindsay popped, but the ex-Steelers coach took a hands on approach with Lindsay's dating life ... at least according to his son-in-law, Ryan.

So, now that they have a baby boy ... the question: Dad is in the NBA, grandpa's an NFL coaching legend -- do you play football or basketball?