The question of Brown’s nick-name and the date at which he acquired it is unlikely to be brought to a final answer. Nor I think should it exercise us overmuch. However Dr D of Cambridge has submitted a newspaper report from The Norfolk Chronicle (29th January 1780) p.4, which has the report of a meeting in Huntingdon at which ‘Mr Browne (a young counsellor Son of Capability Browne who was brought down by Lord Sandwich in his own coach for the purpose)’ stepped forward to give his opinion of a petition.
Such 18th century nicknames are not common – though the architect ‘Athenian’ Stuart had one, ‘Stiff’ was Leadbetter’s proper name – and there is some interest is there not in seeing how rapidly such nicknames take hold?
The young Brown referred to would have been Capability’s son Lance, or Lancelot II. Lancelot II enjoyed the patronage of Lord Sandwich and went on to become an MP.