Preserved in a sealed metal container, or (informally) pre-recorded or pre-prepared rather than fresh or spontaneous.
From tin cans first widely used for food preservation in the 1800s. Evolved to mean anything preserved/packaged, then metaphorically applied to anything pre-made and not fresh.
The reason 'canned laughter' sounds so annoying is the same reason canned food tastes different—both have been artificially preserved and packaged, losing the fresh, spontaneous quality of the original.
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