Emily Dickinson Lexicon

LAG, v.i. [W. llag, llac, slack, loose; Goth. laggs, long; Eng. to flag, and flacceo, langueo, to languish, &c. The sense is to extend or draw out, or to become lax or loose. Class Lg.]

To walk or move slowly; to loiter; to stay behind. I shall not lag behind. – Milton.