Definition of an heir and a spare in English:

an heir and a spare

phrase

British
humorous
  • Two children (typically in reference to members of the monarchy or nobility, said to need two children, one to succeed to a title and the other to guarantee the family line should anything happen to the first):

    ‘the future King wants to follow in his parents' footsteps by producing the minimum royal issue of an heir and a spare’
    • ‘He had the requisite heir and a spare but unfortunately all three had died in a boating accident.’
    • ‘The latest edition to the heir and a spare Royal rank, the baby will replace Prince Harry as fourth in line to the throne.’
    • ‘Her job, just like others before her, is to produce an heir and a spare.’
    • ‘Although about half have provided an heir and a spare to inherit their fortunes, almost half have not even seen the inside of a maternity ward.’
    • ‘She did have two kids with Charles - an heir and a spare.’
    • ‘In those days of high infant mortality, it was not enough to have one son, or even an 'heir and a spare' as the British say.’
    • ‘She was purely there to produce an heir and a spare.’