E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Against the collar. Somewhat fatiguing. When a horse travels up-hill the collar distresses his neck, so foot-travellers often find the last mile or so against the collar, or distressing. Authors of long books often find the last few pages wearisome and against the grain.
In collar. In harness. The allusion is to a horses collar, which is put on when about to go to work.
Out of collar. Out of work, out of place. (See above.)
To slip the collar. To escape from restraint; to draw back from a task begun.
To work up to the collar. To work tooth and nail; not to shirk the work in hand. A horse that lets his collar lie loose on his neck without bearing on it does not draw the vehicle at all, but leaves another to do the real work.
As regarded himself, the path lay plain. He must work up to the collar, hot and hard, leaving himself no time to feel the parts that were galled and wrung.Mrs. Edwardes: A Girton Girl, chap. iv.