Have you transcribed it correctly?

The Irish alphabet doesn't contain the letters Q, Z or J

But the following website suggests that it means much the same as "quo vadis" (whither goest thou, where are you going)

http://www.clanstewart.org/History/ClanBranches.asp

Clan Stewart of Appin
Gaelic Name Mac-Iain Stiuibhairt na h-Apunn
English Names Stewart
Lands Appin, in the west Highlands.
Crest Badge A unicorn's head, crined and armed, or.
Plant Badges Oak or Thistle
Mottos "Quidder we'll zje"
"Whither will ye."
War Cry "Creag ab Sgairibh"
"The cormorant's rock"
Clan History

Being one of the western Highland branches of Clan Stewart, this Clan was on terms of friendship with Clan MacLaren; they supported each other in several battles.

The banner of the Stewarts of Appin Regiment was one of the few saved from destruction after the Battle of Culloden.