Regarding Ann Rutan's Aug. 20 letter chastising President Reagan's use of "fellow" in the phrase "My fellow Americans," I wish to point out the following:

Webster's Ninth New Collegiate Dictionary, copyright 1985, page 455, defines the use of the word "fellow" as: "1: COMRADE, ASSOCIATE 2a: an equal in rank, power or character: PEER b: one of a pair: MATE 3: a member of an incorporated literary or scientific society 4a obs: a person of one of the lower social classes b: a worthless man or boy c: MAN, BOY d: BOYFRIEND, BEAU 5: an incorporated member of a college or collegiate foundation esp. in a British university 6: a person appointed to a position granting a stipend and allowing for advanced study or research." The three principal definitions make no mention of gender, while the fourth definition is considered obsolete. Grouping the meaning of the three principal definitions into one encompassing definition, the word means "members of the same group." I believe this is the context in which Mr. Reagan is using "fellow."

In addition, I polled 27 females in my office. None of them feels "very much left out" by "My fellow Americans."

CHRIS BERDUX

Falls Church

Please! I suggest that before Ann Rutan writes another letter to the editor, she first check her dictionary.

According to Funk & Wagnalls Standard Dictionary, "fellow" is defined first as ''a man or boy," but the three following definitions are: 1) ''a person; anybody; one"; 2) ''a comrade or companion''; 3) ''an individual belonging to the same kind, class or group as oneself.'' I believe she must fall into one of the aforementioned categories.

Oh, and while she has her dictionary out, might I suggest she also look up piddling: ''unimportant; trivial, trifling.''

CYNTHIA HEIN

Crofton, Md.

Yes, Ann Rutan, I understand why you feel left out by the president's salutation ''My fellow Americans,'' although fellowship in the general sense includes both women and men. While the adjective means being a companion, mate or associate, and noun usage can indicate a person of either sex, the word has come to be used chiefly to refer to men. I suggest ''My associate Americans,'' a more formal phrase, but one without doubt inclusive.

MARIE ANNE ERICKSON

Braddock Heights, Md.

I am a graduate fellow at the University of Maryland. It never occurred to me to resent being called a "fellow." On the contrary, I am very pleased to be one.

However, after I read Ann Rutan's letter I consulted several dictionaries. The word seems to be derived from an Old English term for business partner. Therefore, I think the president's use of the word is apt.

ANN WASS

Riverdale