Harden, season, acclimatize, acclimate denote to make (as a person) proof against hardship, strain, or exposure. All imply a becoming accustomed or adapted by time or experience.
Harden implies habituation that toughens one and makes one insensible of one’s own pain or discomfort or callous and insensitive to others’ misery.
Season implies a gradual bringing into mature, sound, efficient condition; it does not, when referred to persons, necessarily imply that what is to be undergone is uncongenial.
Acclimatize and acclimate imply adaptation to a new and adverse climate or, by extension, to new and strange surroundings in general.
Some writers have distinguished acclimatize from acclimate by restricting the first to adaptation by human agency, but this distinction is not commonly observed.