The phrase "be going soft (in the head)" means that someone is losing their mental sharpness or becoming less intelligent. There are many synonyms for this phrase, including "losing one's marbles," "senile," "dementia," "feeble-minded," "unintelligent," "not all there," "brain-dead," "dim-witted," "mental decline," "forgetful," and "scatterbrained." These synonyms all suggest a decline in cognitive abilities and can be used interchangeably to describe someone who is losing their mental faculties. It's important to remember that these phrases are not meant to be offensive and should be used with care and compassion when discussing mental health.