Means your heart is not protected. It’s out there where others can bump, bruise, and sneeze on it. Open to heart-break.

It has a secondary connotation of one who is without guile, and who adores openly, without shame.

The term comes from Vaudeville, and the costumes used:

The practice of pulling Handkerchiefs out of sleeves (wrist cuffs) to wipe eyes for effect predates Vaudeville, but was used to ridiculous effect in Vaudeville.

“BooHooHoo, the Heroine cried”, and all that.

The phrase is a rather cynical one, as are most Vaudeville phrases (Pig Show, Dog and Pony, Brat Spank, TDTGO, etc.), but it has endured to this day to mean Honest.