The words are "jacking gown," whose meaning I don't know.

From Robert Ford's Vagabond Songs and Ballads of Scotland [first series] (1899, pp. 236-38):

    BONNIE JOHNNIE LOWRIE

X:1
T:BONNIE JOHNNIE LOWRIE
M:4/4
L:1/8
K:G
B|B e e d B G d B|B e e f g3/2 e/ f
w:Of a' the lads in Tin-wald town, The love-ly fair, or black, or brown,
d|B/ e3/2 e d B G d3/2 B/|B/ e3/2 d B A2 A z||
w:There nev-er was sae droll a loon As bon-nie John-nie Low-rie.
G/ E3/2 E G D E GA|G/ E3/2 E F G A B3/2
w:Ter-rie ow-den dow-den day,_ Ter-rie ow-den dow-den day,
A/|G/ E3/2 E G D3/2 E G A|B/ e3/2 d B A2 A||
w:The queer-est loon in laigh or brae, Is bon-nie John-nie Low-rie.

Of a' the lads in Tinwald town
The lovely fair, or black or brown,
There never was sae droll a loon
    As bonnie Johnnie Lowrie.

    Tirrie owden, dowden dow, etc. [sic]

My dad a peck o' lint did sow,
I gaed to see how it did grow,
When wha come skipping owre the knowe,
   But bonnie Johnnie Lowrie.

I wandered oot to weed the same,
My laddie ken'd I was frae hame;
To follow me he wasna lame,
   My bonnie Johnnie Lowrie.

I took the flax unto the mill,
My jewel follow'd after still;
And coming hame I gat a gill
   Frae bonnie Johnnie Lowrie.

When I gaed to the Bar to shear,
Close at my heels I had my dear;
I in the kemp the gree did bear
   Wi' bonnie Johnnie Lowrie.

And when I went to the Rood-fair,
I wat I didna want my share
O' a' the good things that were there,
   Frae bonnie Johnnie Lowrie.

At last, a'e nicht, into the park,
I met wi' him when it was dark,
And, oh, the kissin' that I gat
   Frae bonnie Johnnie Lowrie!

But Johnnie's true; he did me wed,
Yestreen before the priest we gaed;
I carena noo for mam or lad,
   Sin' I hae Johnnie Lowrie.

A fig, say I, for jacking gown,
Or priest or Elder in the toun;
I'll tak' the warld, rough and roun',
   Wi' bonnie Johnnie Lowrie.

Until recently I knew this song by name only, though, by name, very well; and the copy here presented--with slight modifications--I discovered in a chapbook, printed at Kilmarnock (no date), which contains besides "Willie was a Wanton Wag," "Bailie Nicol Jarvie's Journey to Aberfoyle," "Whistle and I'll come to you, my lad," and "Scottish Whiskie." The mention of Tinwald in the opening line marks the song as presumably a native of Dumfries-shire.

A related version titled "Johney Lowre" ("Of all the lads in Glasgow town ...") is at Bodleian Library Broadside Ballads.

johney lowre [title]