From English dialectal slat (“to throw or dash about”), from Middle English sclatten, skleatten (“to flap”), from Old English slǣtan (“to sleat, slate”) and/or Old Norse sletta (“to slap, dash”) + -er.
slatter (third-person singular simple present slatters, present participle slattering, simple past and past participle slattered) (intransitive)
Part or all of this entry has been imported from the 1913 edition of Webster’s Dictionary, which is now free of copyright and hence in the public domain. The imported definitions may be significantly out of date, and any more recent senses may be completely missing.
(See the entry for “slatter”, in Webster’s Revised Unabridged Dictionary, Springfield, Mass.: G. & C. Merriam, 1913, →OCLC.)