The difference between Stag and Stag do

When used as nouns, stag means an adult male deer, whereas stag do means a stag party.

Stag is also adverb with the meaning: of a man, attending a formal social function without a date.

Stag is also verb with the meaning: to act as a "stag", an irregular dealer in stocks.

check bellow for the other definitions of Stag and Stag do

Stag as a noun:

An adult male deer.
Stag as a noun:

A colt, or filly.
Stag as a noun (by extension, obsolete):

A romping girl.
Stag as a noun:

An improperly or late castrated bull or ram – also called a bull seg (see note under ox).
Stag as a noun:

An outside irregular dealer in stocks, who is not a member of the exchange.
Stag as a noun:

One who applies for the allotment of shares in new projects, with a view to sell immediately at a premium, and not to hold the stock.
Stag as a noun:

The Eurasian wren, Troglodytes troglodytes.
Stag as a noun (usually, _, attributive):

An unmarried male, a bachelor; a male not accompanying a female at a social event.

Examples:

"a stag dance; a stag party"
Stag as a noun:

A social event for males held in honor of a groom on the eve of his wedding, attended by male friends of the groom, sometimes a fund-raiser.

Examples:

"The stag will be held in the hotel's ballroom."
Stag as a noun:

A stag beetle (family ).

Stag as a verb (intransitive, British):

To act as a "stag", an irregular dealer in stocks.
Stag as a verb (transitive):

To watch; to dog, or keep track of.

Stag as an adverb:

Of a man, attending a formal social function without a date.

Examples:

"My brother went stag to [[prom]] because he couldn't find a date."