Definition of "first thing"

Some of 'em said that when a boy had been away from home a couple of years he ought to want to see his folks the first thing.

1916 March 11, Charles E. Van Loan, “His Folks”, in Saturday Evening Post‎

I'll do it first thing after school tomorrow.

2009, Alex J. Packer, Bringing Up Parents: The Teenager's Handbook

You had to see it first thing, 'cause it was the first thing that caught the eye.

2012, Cd Harper, And Face the Unknown: The Journey of a Lincoln-freed Colored

Whatever it is, getting it out of the way is crucial to your forward momentum. So, to borrow a phrase from Nike, just do it—and do it first thing.

2013, Nelson Searcy, Jennifer Dykes Henson, The Renegade Pastor: Abandoning Average in Your Life and Ministry, page 2013

I stumbled along through the young pines and huckleberry bushes. Pretty soon I struck into a sort of path that, I cal'lated, might lead to the road I was hunting for. It twisted and turned, and, the first thing I knew, made a sudden bend around a bunch of bayberry scrub and opened out into a big clear space like a lawn.

1913, Joseph C. Lincoln, chapter 1, in Mr. Pratt's Patients