Too much caffeine gave him a noticeable twitchiness, making his fingers tap nervously on the table.

twitchiness 是由动词 twitch（抽动、抽搐）加上后缀 -iness 构成的名词。twitch 表示"突然的抽动或痉挛动作"，后缀 -iness 表示"具有某种性质或状态"。twitchiness 合起来表示"神经质的抽动状态"，即"紧张不安"。