What's the intuition behind the sin of an obtuse angle? The definition of sin of an angle is opposite / hypotenuse. Obviously, this definition is easy to use for acute angles within a right triangle, but it's hard to see how it carries over for obtuse angles.

I know there are trig identities and unit circle concepts that allow one to get to the sin of an obtuse angle, but I'm looking for a more intuitive explanation for what exactly the sin of an obtuse angle means.
"The definition of sin of and angle is opposite/hypotenuse" - well that only works for a right-angled triangle, and is the beginning of the definition of the sine function.

In order to extend the definition draw a unit circle with centre at the origin. Measure the angle counterclockwise from the positive $x$-axis and take a point $(x,y)$ on the circle. The sine of the angle (equivalent to opposite/hypotenuse in the first quadrant, with the hypotenuse made equal to $1$) is $y$ and the cosine of the angle is $x$. You can go round the circle more than once, so you can see that the functions are periodic.

This is why the functions are sometimes known as circular functions and underlies why they come in so surprisingly useful.



In case you are interested ...

In more advanced work the sine function is sometimes defined very differently, with the angle measured in radians ($2\pi$ radians $=360^{\circ}$). Then $$\sin x = \sum_{r=1}^\infty (-1)^{r-1}\frac{x^{2r-1}}{(2r-1)!}=\frac {e^{ix}-e^{-ix}}{2i}$$This can be applied in more general circumstances still, and the series is convenient because it converges rapidly and enables the sine function to be computed accurately for practical use. The first few terms are $$x-\frac {x^3}6+\frac {x^5}{120}-\frac {x^7}{5040}+\dots$$
Let's use the definition of the slope or tangent of the terminal line for each angle ....
For the terminal line of an acute angle the slope is y/x....
For the terminal line of an obtuse angle the slope is also y/x.....(though negative in sign)....
Now we multiply each slope by x to get us the sin values for the two angles (in the unit circle ) as y and -y which are equal in absolute values.....