An Unstrung adamant crossbow is an adamant crossbow before it is strung. Unstrung adamant crossbows can be made through the Fletching skill after achieving level 61 by using a mahogany stock with adamant crossbow limbs. This grants 82.0 Fletching experience. A player can finish the crossbow by adding crossbow string, granting 41 experience and making the unstrung adamant crossbow into an adamant crossbow. This item can also be obtained as a drop from a Skeletal wyvern.
Dropping monsters
For an exhaustive list of all known sources for this item, see here.