Probably an alteration of the obsolete signet (“signal”)
sennet (plural sennets)
- a signal call given on a cornet or trumpet for entrance or exit on a theatrical stage
- The barracuda.
- “sennet” in Webster’s Seventh New Collegiate Dictionary: Based on Webster’s Third New International Dictionary, 7th edition, Springfield, Mass.: G[eorge] & C[harles] Merriam, 1963 (1967 printing), OCLC 974117641.