You're right that it originates in the mil but I don't know where it comes from. I don't think it's widely used even in the mil although it's probably more common at early training stations and Univesity Air Squadron home airfields.
CAP 413, the RT Manual, Chapter 9 Page 3 says "Inexperienced civil pilots are invited to use the callsign prefix ?TYRO? when in communication with a military unit or the D&D Section to indicate their lack of experience. Upon hearing this code word, military controllers will ensure that they do not issue complex instructions which the pilot could have difficulty in following.
Certainly some civi controllers recognise the term but by no means all and the RT manual limits its use to mil units. My advice would be to use other standard phrases such as 'speak slower' or 'say again' if you're having trouble with the RT.