The Collaborative International Dictionary of English v.0.48:
Unburden \Un*bur"den\, v. t. [1st pref. un- + burden.] 1. To relieve from a burden. [1913 Webster] 2. To throw off, as a burden; to unload. [1913 Webster]WordNet (r) 3.0 (2006):
unburden v 1: free or relieve (someone) of a burden 2: take the burden off; remove the burden from; "unburden the donkey" [syn: unburden, disburden] [ant: burden, burthen, weight, weight down]