In computer geek parlance, to crowbar something is to gain access to it in a manner which bypasses the ordinary security measures intended to stop people from gaining administrative access. This could range from shorting the CMOS clear jumper to clear a BIOS password to putting a serial console on a Cisco router in order to change the configuration. As these examples illustrate, the most common cause of crowbarring is a lost password, either due to forgetting it, or due to an admin departing a company.
Generally speaking, networking devices are crowbarred by the use of a serial console. Some of them are passwordless for a few seconds after being powered on; some can be sent into a diagnostic monitor by sending a break during POST/boot and you can then change a configuration register in order to disable security. Computers, on the other hand, are usually crowbarred either at the boot prompt by sending a machine into single-user mode, or by booting (via floppy disk or CD-ROM) the same operating system as on the machine (or sometimes a different one) and changing some files in order to change a password.