desu shi ですし

In Japanese, desushi ですし is the polite copula desu です plus the shi し particle. It works the same way as dashi だし, which has the plain copula da だ instead.

• tetsudatte-kuremasu ka?
  手伝ってくれますか？
  Will [you] help [me]?
• ii desu yo, douse hima desu shi
  いいですよ、どうせ暇ですし
  Fine, since [I] have nothing to do anyway.

The only different, as usual, is when dealing with i-adjectives.

• kawaii shi
  可愛いし
  Since [it] is cute.
• kawaii desu shi
  可愛いですし
  (polite variant of the above.)
• kawaii da shi
  可愛いだし
  (this is wrong, because da だ can't come immediately after an i-adjective, even though desu です can.)