Definition of first thing
- I'll be back first thing (on) Monday morning.
- She promised to call first thing in the morning.
: preceding all others in time, order, or importance: such as
: being the lowest forward gear or speed of a motor vehicle
: before another in time, space, or importance
: in the first place
: for the first time
: one that is number one in a series — see Table of Numbers
: something that is first: such as
: the first occurrence or item of a kind
: an object or entity not precisely designated or capable of being designated
: an inanimate object distinguished from a living being
: a separate and distinct individual quality, fact, idea, or usually entity
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