The difference between Mad and Nutty as a fruitcake

When used as adjectives, mad means insane, whereas nutty as a fruitcake means behaving in an eccentric, foolish, or kooky manner.

Mad is also adverb with the meaning: intensifier.

Mad is also verb with the meaning: to be or become mad.

check bellow for the other definitions of Mad and Nutty as a fruitcake

Mad as an adjective:

Insane; crazy, mentally deranged.

Examples:

"You want to spend $1000 on a pair of shoes? Are you mad?"

"He's got this mad idea that he's irresistible to women."
Mad as an adjective (chiefly, US; UK dated + regional):

Angry, annoyed.

Examples:

"Are you mad at me?"
Mad as an adjective:

Wildly confused or excited.

Examples:

"to be mad with terror, lust, or hatred"
Mad as an adjective:

Extremely foolish or unwise; irrational; imprudent.
Mad as an adjective (colloquial, usually with ''for'' or ''about''):

Extremely enthusiastic about; crazy about; infatuated with; overcome with desire for.

Examples:

"Aren't you just mad for that red dress?"
Mad as an adjective (of animals):

Abnormally ferocious or furious; or, rabid, affected with rabies.

Examples:

"a mad dog"
Mad as an adjective (slang, chiefly Northeastern US):

Intensifier, signifies an abundance or high quality of a thing; very, much or many.

Examples:

"I gotta give you mad props for scoring us those tickets. Their lead guitarist has mad skills. There are always mad girls at those parties."
Mad as an adjective (of a compass needle):

Having impaired polarity.

Mad as an adverb (slang, New England, New York, and, UK, dialect):

Intensifier; to a large degree; extremely; exceedingly; very; unbelievably.

Examples:

"He was driving mad slow."

"It's mad hot today."

"He seems mad keen on her."

Mad as a verb (obsolete, intransitive):

To be or become mad.
Mad as a verb (now, _, colloquial, _, US):

To madden, to anger, to frustrate.

Nutty as a fruitcake as an adjective (simile, colloquial):

Behaving in an eccentric, foolish, or kooky manner; very nutty.