Just so it’s clear, Avengers: Infinity War did crafts backstory for the MCU’s Thanos.
However, as the Russos told Total Film Magazine, none of this will be outright shown during Infinity War.
That said, with Josh Brolin reprising the purple-skinned alien warlord in Avengers 4 next year, perhaps that’s where his past will be directly explored.
Thanos certainly won’t be lacking for screen time, as the directing duo reaffirmed that the movie is being “told from his point of view”; we just won’t see his origin story depicted.