Trusty, a. [Compar. Trustier; superl. Trustiest.]
1. Admitting of being safely trusted; justly deserving confidence; fit to be confided in; trustworthy; reliable. Your trusty and most valiant servitor. Shak.
2. Hence, not liable to fail; strong; firm. His trusty sword he called to his aid. Spenser.
3. Involving trust; as, a trusty business. [R.] Shak.
Shades of meaning
Fernald has no entry of its own for trusty; he lists it under faithful, honest, reliable.
Where he writes about it:
A person is faithful who will keep faith, whether with or without power to aid or serve; a person or thing is trusty that possesses such qualities as to justify the fullest confidence and dependence. — from faithful
We may speak of a faithful but feeble friend; we say a trusty agent, a trusty steed, a trusty sword. — from faithful
Trusty and trustworthy refer to inherent qualities of a high order, trustworthy being especially applied to persons, and denoting moral integrity and truthfulness; we speak of a trusty sword, a trusty servant; we say the man is thoroughly trustworthy. — from reliable
A reliable messenger is one who may be depended on to do his errand correctly and promptly; a trusty or trustworthy messenger is one who may be admitted to knowledge of the views and purposes of those who employ him, and who will be faithful beyond the mere letter of his commission. — from reliable