Green’s Dictionary of Slang

Navvy n.

also navvy
[abbr.]
(US, mainly Southwest)

1. a Navaho Indian.

[US]Z. Grey Heritage of Desert (1937) 10: Seen any Navvies?
A. Bean Fancher Train 28: The Navvies are damn smart when it comes to breedin’ [HDAS].
J.U. Terrell Bunkhouse Papers 184: They’re a no-good bunch of Navvies up around there [HDAS].

2. a Navaho pony.

[US]R.F. Adams Cowboy Lingo 86: The ‘navvy’ was a Navajo Indian pony, said to be about the poorest specimen of horse-flesh on earth.
[UK]A. Sillitoe ‘Noah’s Ark’ Loneliness of the Long-Distance Runner (1960) 98: As though he were a tent from which ten buck-navvies were trying to escape.