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.

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

Source: Webster's Revised Unabridged Dictionary (1913)
 

Simmer \Sim"mer\, v. t. To cause to boil gently; to cook in liquid heated almost or just to the boiling point.

Source: Webster's Revised Unabridged Dictionary (1913)