The word 头昏脑胀 is formed by combining two two-character phrases: 头昏, meaning "head dizzy", and 脑胀, meaning "brain swollen". Each phrase describes a physical sensation, 头 (head) with 昏 (dizzy) and 脑 (brain) with 胀 (swollen), and together they create a compounded effect to emphasize a state of mental fogginess or discomfort, typically from illness or fatigue. The formation relies on the parallel and synonymous nature of the phrases to reinforce the overall meaning.