Ven. Ñanamoli translated nibbida as "estrangement". He has a lengthy note on the word in his Three Cardinal Dicourses :
ESTRANGEMENT: the Pali noun nibbida and its verb nibbindati are made up from the prefix nir in its negative sense of “out,” and the root vid (to find, to feel, to know intimately). Nibbida is thus a finding out. What is thus found out is the intimate hidden contradictoriness in any kind of self-identification based in any way on these things (and there is no way of determining self-identification apart from them — see under NOT-SELF). Elsewhere the Buddha says:
Whatever there is there of form, feeling, perception, determinations, or consciousness, such ideas he sees as impermanent, as subject to pain, as a sickness, as a tumour, as a barb, as a calamity, as an affliction, as an alienation, as a disintegration, as void, as not-self. He averts his heart from those ideas, and for the most peaceful, the supreme goal, he turns his heart to the deathless element, that is to say, the stilling of all determinations, the relinquishment of all substance, the exhaustion of craving, the fading of passion, cessation, extinction. (MN64)
The “stuff” of life can also be seen thus. Normally the discovery of a contradiction is for the unliberated mind a disagreeable one. Several courses are then open. It can refuse to face it, pretending to itself to the point of full persuasion and belief that no contradiction is there; or one side of the contradiction may be unilaterally affirmed and the other repressed and forgotten; or a temporary compromise may be found (all of which expedients are haunted by insecurity); or else the contradiction may be faced in its truth and made the basis for a movement towards liberation. So too, on finding estrangement thus, two main courses are open: either the search, leaving “craving for self-identification” intact, can be continued for sops to allay the symptoms of the sickness; or else a movement can be started in the direction of a cure for the underlying sickness of craving, and liberation from the everlasting hunt for palliatives, whether for oneself or others. In this sense alone, “Self protection is the protection of others, and protection of others self-protection” (Satipatthana Samyutta).
"Perfect Ones do not smile for no reason." Ghaṭīkāra Sutta