Short answers, pulled from the story.
In Buddhism, dukkha is one of the three marks of existence alongside anicca (impermanence) and anatta (no lasting essence), and it forms the basis of the first of the Four Noble Truths. Buddhist analysis divides it into three layers: dukkha-dukkha (plain physical and mental suffering), viparinama-dukkha (the frustration of pleasant experiences ending), and sankhara-dukkha (the pervasive unsatisfactoriness of all conditioned, impermanent existence).
Two derivations compete. The more popular traces dukkha to du- ("bad" or "difficult") and kha ("empty hole"), specifically the axle hole of a cart wheel: a poorly fitting axle produces a rough, bumpy ride. Monier Monier-Williams and Analayo argue the actual Sanskrit roots are dus- ("bad") and stha ("to stand"), meaning "standing badly" or "unsteady", with phonological shifts in the Prakrits producing the form dukkha.
No single English word covers the full range of dukkha. Translators before the 1970s typically used "suffering", but later scholars considered that too narrow. Modern translators use terms such as anxiety, distress, frustration, unease, or unsatisfactoriness, and many leave the word untranslated. Its meaning shifts with context: it can denote physical pain, the disappointment of impermanent pleasures, or a subtle sense that conditioned life never fully satisfies.
Dukkha appears in the earliest Upanishads, including the Brhadaranyaka and Chandogya, in discussions of spiritual liberation through knowledge of Atman. It also occurs in the Bhagavad Gita, the Shvetashvatara Upanishad (verse 6.20), and in the foundational sutras of schools such as Samkhya, whose text opens by addressing suffering directly. The Yoga Sutras of Patanjali state that "for one who has discrimination, everything is suffering" (dukkham eva sarvam vivekinah).
The direct opposite of dukkha is sukha, meaning lasting happiness, comfort, or ease. In its original Aryan context, sukha meant "having a good axle hole" while dukkha meant "having a poor axle hole". In Buddhist usage, sukha (pleasure) and dukkha (unpleasantness) are treated as opposite poles of feeling, yet both are regarded as impermanent and therefore ultimately unsatisfactory.
Dukkha is addressed in the Tattvartha Sutra, an authoritative Jain scripture from the 2nd century. Jainism places the concept within its own framework of karma and liberation, distinct from the Buddhist analysis of craving and the Hindu emphasis on Atman-knowledge, while sharing the same Sanskrit linguistic heritage.