What is the exact meaning of "manoeuvring" when said by ATC in reference to traffic?
To add to @HiddenWindshield's answer, it's explained in UK's CAA Radiotelephony Manual CAP 413:

Traffic Information and Avoiding Action Phraseology
5.21 Relative movement should be described by using one of the following terms as applicable:
(...)
5. 'manoeuvring' where the conflicting traffic’s flight path and/or level information is unpredictable and/or showing significant variation.