Don Lemon -- Defends Ferguson Pot Shot ... The Guy Was Smoking a Joint!

Don Lemon is baffled why his remark about Ferguson "obviously" smelling like marijuana offended anyone -- but he does explain why he said it in the first place.

The CNN anchor chuckled at our photog Sunday night when we mentioned his recent guffaws -- telling a Bill Cosby rape accuser she could have avoided the situation ... and saying the Ferguson protests "obviously" smell like marijuana.

Lemon says someone had just walked past him smoking a joint ... so he just blurted out the comment. Watch ... it's obvious Don still doesn't get why the word "obvious" rubbed people the wrong way.