More buggy; more infested with bugs, more unreliable in function, or resembling a buggy more.
From 'buggy' (infested with bugs, unreliable, or resembling a buggy) plus '-er' (comparative suffix).
In software development, 'buggy' became the standard term for code with errors, making 'buggier' the programmer's way of comparing how broken two pieces of code are.
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