The full form of FWIW is For What It’s Worth. It’s used on Miscellaneous ,Chat slang in Worldwide

“For What It’s Worth (Stop, Hey What’s That Sound)” (often referred to as simply “For What It’s Worth”) is a song written by Stephen Stills. Performed by Buffalo Springfield, it was recorded on December 5, 1966, released as a single on Atco Records on December 23, 1966, and peaked at No. 7 on the Billboard Hot 100 chart in the spring of 1967.

[Verse 1]
There’s something happening here
But what it is ain’t exactly clear
There’s a man with a gun over there
A-telling me, I got to beware

[Chorus]
I think it’s time we stop
Children, what’s that sound?
Everybody look what’s going down

[Verse 2]
There’s battle lines being drawn
And nobody’s right if everybody’s wrong
Young people speaking their minds
Are gettin’ so much resistance from behind

[Chorus]
It’s time we stop
Hey, what’s that sound?
Everybody look what’s going down

[Verse 3]
What a field day for the heat (Ooo-ooo-ooo)
A thousand people in the street (Ooo-ooo-ooo)
Singing songs and a-carryin’ signs (Ooo-ooo-ooo)
Mostly say, “Hooray for our side” (Ooo-ooo-ooo)

for what it’s worth