“beauty which you hold in lease”
A lease gives someone the right to use property for a fixed term, after which the term expires and the property must be returned or renewed. The poet argues that the Youth does not own his beauty outright — he holds it for the length of his life only, like a tenant, not a freeholder. At death the lease runs out and the beauty is lost unless a child carries it forward.
historical The procreation argument (Sonnets 1-17)“Find no determination; then you were”
'Determination' here is a legal term meaning the ending or expiry of a lease or term. The sense is: if the Youth passes his beauty to a child, that beauty will find no 'determination' — no legal end-date, no expiry. The line continues the lease conceit of the previous line: a child effectively renews the lease beyond one lifetime.
“Who lets so fair a house fall to decay”
At line 9 the poet pivots from abstract legal argument (the lease/determination conceit) to a concrete domestic question: who would let a fine house fall to ruin when good management could preserve it? This is the sonnet's volta, the turn of thought. 'House' carries two meanings at once: the physical building and the family lineage ('the house of York,' 'the house of Tudor'). Letting it 'fall to decay' means both failing to maintain an estate and dying without an heir.
historical The sonnet form“husbandry in honour might uphold”
'Husbandry' meant both the careful management of a farm or estate and, by derivation from 'husband,' the duties proper to a married man. The word compresses the poem's whole argument into a single noun: to be a good steward of your beauty is inseparable from being a husband and fathering children. The word 'honour' adds a third pressure — to let beauty die without an heir is not merely wasteful but dishonourable.
historical The procreation argument (Sonnets 1-17)“O! none but unthrifts”
An 'unthrift' is a spendthrift or wastrel — someone who squanders rather than manages what they have been given. The poet's indignant answer to his own rhetorical question ('Who lets so fair a house fall to decay?') is: no one but an unthrift would. Calling the Youth an unthrift is mild but pointed: it reframes not marrying as a failure of basic household economy, not a romantic preference.