Nightmares and Daydreams

Vivid colloquial image. The literal reading of 'ball of nerves' does not convey the full idiomatic meaning of a person in a constant state of anxiety.

Used literally in this dialogue but also used ironically in everyday speech to mock someone acting entitled or superior.

Used mockingly when someone complains about mild physical discomfort. The image is transparent in isolation, but its use as a taunt about toughness is idiomatic.

Used here as a friendly nickname. In everyday speech, body-part nicknames like this are common terms of affection or teasing among friends.