According to a (non japanese, but he spent lots of time in Japan) Aikido teacher I know, this is supposed to represent "an obligation bounding student and teacher".
To be honest, I am a bit skeptical. I know that this is the starting part of the name of a specific "Kata" in Iwama Style Aikido (see here for an example) but I wonder if there is some other "esoteric" meaning or figure of speech that I am missing.