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通用的解释
** rc= RunCom =Run commands**

通常指包含命令启动信息的文件。

"rc" 是取自 "runcom", 来自麻省理工学院在 1965 年发展的 CTSS系统。相关文献曾记载这一段话: '具有从档案中取出一系列命令来执行的功能；这称为 "run commands" 又称为 "runcom"，而这种档案又称为一个 runcom (a runcom)。

wiki的解释

The term rc stands for the phrase "run commands". It is used for any file that contains startup information for a command. It is believed to have originated somewhere in 1965 from a runcom facility from the MIT Compatible Time-Sharing System (CTSS).
From Brian Kernighan and Dennis Ritchie: "There was a facility that would execute a bunch of commands stored in a file; it was called runcom for 'run commands', and the file began to be called 'a runcom'. rc in Unix is a fossil from that usage."[1]
Tom Van Vleck, a Multics engineer, has also reminisced about the extension rc: "The idea of having the command processing shell be an ordinary slave program came from the Multics design, and a predecessor program on CTSS by Louis Pouzin called RUNCOM, the source of the '.rc' suffix on some Unix configuration files."

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个人感觉比较好，也比较认同的解释
rc=Run Control 运行控制

rc may also be expanded as "run control", because an rc file controls how a program runs. For instance, the editor Vim looks for and reads the contents of the .vimrc file to determine its initial configuration. In The Art of Unix Programming, Eric S. Raymond consistently refers to rc files as "run-control" files.