The trade of "slathener" or "slatherner"?

[Joel S. Berson]

What was a "slathener" or "slatherner"?  (Both spellings
appear.)  It's some kind of skill for which an indenture was drawn up
for an apprentice in 1676 New England.

Not in OED2 or on the web.  Even "slather" (verb and noun) shows only
the usual senses, including various sexual applications.  And the
German verb "slathen" simply means "to slather"; "schmieren".

Joel