The Collaborative International Dictionary of English v.0.48:

Simmer \Sim"mer\, v. i. [imp. & p. p. Simmered; p. pr. & vb.
   n. Simmering.] [Prov. E. also simper; -- an onomatopoetic
   word.]
   To boil gently, or with a gentle hissing; to begin to boil.
   [1913 Webster]

         I simmer as liquor doth on the fire before it beginneth
         to boil.                                 --Palsgrave.
   [1913 Webster]