Neither Moss nor sand

[shiznit]

My Mum has been using this for ages, anyone know what this jibberish means, if anything at all

[bardotagogo]

Moss is often used as a metaphor for 'that which can not be moved', as in 'A Rolling Stone Gathers No Moss'. Contrariwise, sand is often used as a metaphor for the ephemeral, as in 'shifting sand' or 'letting sand slip through your fingers'.
In your mum's expression, moss and sand therefore come to symbolise (or mean) absolute opposites, so if your mum uses the expression then it simply comes to mean 'not under any circumstances.'

[bardotagogo]

PS Doesn't your mum know, or haven't you asked her?