In the word disinterestedly, the root morpheme is

interest

To determine the root morpheme in the word "disinterestedly," we can break it down into smaller units:

1. "Dis-" is a prefix that means "not" or "opposite of."
2. "Interested" is the base word, derived from "interest," which means "to have or show curiosity or concern about something."
3. "-Ed" is a suffix that forms the past participle tense of the verb "interest."
4. "-Ly" is an adverb-forming suffix that indicates manner or state.

Therefore, the root morpheme in "disinterestedly" is "interest" because it is the base word to which the prefixes and suffixes are added.