1 sincerely, genuinely (am truly grateful).
2 really, indeed (truly, I do not know).
3 faithfully, loyally (served them truly).
4 accurately, truthfully (is not truly depicted; has been truly stated).
5 rightly, properly (well and truly).
Etymology: OE treowlice (as TRUE, -LY(2))
Useful english dictionary. 2012.