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fossil: n.

1. In software, a misfeature that becomes understandable only in historical context, as a remnant of times past retained so as not to break compatibility. Example: the retention of octal as default base for string escapes in C, in spite of the better match of hexadecimal to ASCII and modern byte-addressable architectures. See dusty deck.

2. More restrictively, a feature with past but no present utility. Example: the force-all-caps (LCASE) bits in the V7 and BSD Unix tty driver, designed for use with monocase terminals. (In a perversion of the usual backward-compatibility goal, this functionality has actually been expanded and renamed in some later USG Unix releases as the IUCLC and OLCUC bits.)