"Knockin da Boots" is the title of a 2007 R&B song by H-Town.

Here's a link to that song's lyrics:
http://www.dapslyrics.com/h-town/knockin-da-boots-lyrics/

And here's a link to a YouTube video of that song:

http://www.youtube.com/watch?v=HrBnEaQd4ZY

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What does "knockin da boots" mean?

Well, let's just say that the phrase "knockin da boots" carries on a lusty Blues tradition of creating euphemisms for certain meaningful actions.