1. [adj]  (a boat or vessel) not furnished with official documents
2. [adj]  not registered; "an unregistered citizen"
3. [adj]  (of animals) not recorded with or certified by an official breed association; "unregistered dairy cattle"
4. [adj]  not filed or recorded in the place prescribed by law; "an unregistered trademark"; "an unregistered mortgage"

UNREGISTERED is a 12 letter word that starts with U.