Làirig Dhrù is the hill-pass in the central Cairngorms running between the shoulders of Am Bràigh Riabhach, and Ben Macdui. Although the name is often applied to the entire route between Braemar, and Coylumbridge - I would argue that any ‘pass’ can really only be the summit of the route and that the name can really only apply to that summit.
As a place name Làirig Dhrù is obscure - the derivation has probably been fixed as well as it ever will be, but it's not widely known. In Gordon (1948) - the author gives a good account of its meaning – writing :
As a place-name Lairig Ghru remains an enigma. Lairig means hill pass, and map-makers of the nineteenth century solved the problem to their own satisfaction by substituting for Ghru the word Ghruamach, for which they had apparently not the slightest authority. Ghruamach means forbidding or surly, and forbidding the Lairig often is in wild weather ... But authorities on place-names reject these suggestions, and are obliged to leave the name Ghru a mystery, although it seems to contain the same root as the Allt Dhru burn which drains it to the north. MacBain a distinguished philologist, writes that the name is “probably the Pass of Druie river, from root dru, flow, as in Gaulish Druentia”
– Gordon (1948) (p308)
Seton Gordon's to ‘their own satisfaction’ is probably a reference to the old 6-inch map (1869), where the route is labelled Learg Ghruamach. No doubt Ghruamach was suggested to the map-makers as a likely candidate, and knowing no better the map-makers put that on their map.
Even as early as 1948 then – the weight of suggestion appears to be leaning towards the meaning being related to the water flowing from the floor of the valley close to the watershed. I'm convinced by that, but unconvinced by the suggestion that the burn ‘oozing’ from the pass gave it's name to the pass. That would make the meaning – pass of Druie (it would be the pass of the burn flowing from itself).
Having thought a lot about the derivation, it's my opinion that the name of the pass is the main place name. The pass is pass of oozing - certainly burns ooze from the ground on both sides of the summit. The name of the burn flowing from the pass towards Strath Spey is derived from the name of the pass - not the other way around. Làirig Dhrù is (something like) pass of oozing, and the burn is (something like) burn of pass of oozing.