Jisho does display a "disclaimer" for 氏ぬ: Jocular, humorous term, Internet slang. Intentional use of incorrect kanji.
From what I know, it originated on 2channel. To get around writing the taboo 死ね, a number of replacements were used, like 氏ね or 市ね (for their し reading). タヒね was also used (for its appearance, looking like the bottom half of 死) and タヒる also appears in JMDICT as "Internet slang".