Is this grammatically correct, and what does it mean? “That was the least thing I could do”

The phrase "That was the least thing I could do" is grammatically correct, but it is a bit awkward and doesn't sound completely natural. A more common expression would be "That was the least I could do." In terms of meaning, the phrase suggests that the action taken (implied from the context) was minimal or perhaps required little effort or sacrifice in comparison to what the speaker feels was necessary or appropriate in the situation. Essentially, it conveys a sense of humility or acknowledgment that more could have been done.