Multi-clanning is the act of being in more than one clan at once. Multi-clanning is usually looked down upon, and being caught doing such may be met with an eviction from any clan that the said player would be in.
Most clans don't allow multi-clanning to ensure that members are loyal to only one faction. Less strict groups may allow multi-clanning on approval or without any needed approval. Clans which forbid multi-clanning generally have lower enlistment rates. However, these clans generally have more loyal and active members, because their members devote their time to only one clan.
Generally, it is banned in all major warring ladders and completely frowned upon, if you are in the RSC or RSB.
Why is it bad? Edit
Multi-clanning is generally considered by clans as a bad thing. This is because it means the player may not be as loyal to the clan, as they are in another one. As well as this, some muti-clanners are spies from other clans. Although this is rare, some clans will have spies, who join other clans to get information. Also, when a clan goes to war, if both clans have the same member (due to multi-clanning), they will both want the member to fight for them, though it is only possible by multi-logging which is against the rules.