Definition of needs must (or must needs) do something in US English:

needs must (or must needs) do something

phrase

archaic
  • Cannot avoid or help doing something.

    ‘they must needs depart’
    • ‘Said Ralph, ‘If you will not seek the man, I must needs do it myself; see you then that no harm come to the mare and the coals, and I will look for Wymond, for certainly it was he that bade me come hither.’’
    • ‘They rob man of all the comfort of the Gospel; as if he must needs do something and perform some work to justify his own soul!’
    • ‘Hence the ‘see system,’ as it is called; which simply means that the Bishop who would thus exercise his office must needs do it within definite limits of space, and where the want is greatest, that is, where life and population have met together, in the city, rather than primarily over long lines of railways and interminable prairies.’