Albert Barnes Commentary Joshua 22:19

"Howbeit, if the land of your possession be unclean, then pass ye over unto the land of the possession of Jehovah, wherein Jehovah`s tabernacle dwelleth, and take possession among us: but rebel not against Jehovah, nor rebel against us, in building you an altar besides the altar of Jehovah our God." — Joshua 22:19 (ASV)

Unclean - that is, unholy because the sanctuary was not in it, but on the other side of Jordan.