What is BSL? BSL stands for Bra Strap Length. This makes reference to hair which extends down to the level of the bra strap. POSTS RELATED TO THIS TERM Terminal Length – How Long Can Your Hair Actually Grow? 7 Ways To Embrace The Length Of Your Natural Hair At Any Stage … How To Get 4b Hair To Waist Length In 3 Years TweetPinShare0 Shares Home » Glossary » B » What is BSL?