fidus Achates
Concise Oxford English Dictionary © 2008 Oxford University Press:
fidus Achates /ˌfʌɪdəs əˈkeɪtiːz/
▶noun literary a faithful friend or devoted follower.
– origin L., lit. ‘faithful Achates’ (a companion of Aeneas in Virgil's Aeneid).
'fidus Achates' also found in these Oxford entries:

