Part of speech: Adverb
in a simple manner; without extravagance or embellishment
Example: she was dressed plainly; they lived very simply
Manifestly, Plain, Apparently, Patently, Plainly
The word "plainly" comes from Middle English playnly.
plain – A related word in the same word family.
ly – A related word in the same word family.
Many Middle English words had multiple spellings because standardization didn't exist yet.